Although the influence of thyroid hormones on muscle development and dysfunction has been recognized for a long time, little progress has been made in understanding the site of activity or the mechanism by which it affects the biosynthesis and degradation of muscle proteins. It is the ultimate goal of this study to elucidate the thyroid hormone control mechanisms involved in the control of muscle protein metabolism. Administration of the thyroid hormone triiodothyronine (T3), to premetamorphic bullfrog tadpoles induces precocious hindlimb development and muscle protein synthesis. The specific aim of this study is to identify the mechanism of thyroid hormone action by determining effect of T3 on the following: (1) Non-histone protein composition of chromatin using two dimensional polyacrylamide gel electrophoresis (2D-PAGE) (2) Relative abundance of messenger-RNA (mRNA) for the muscle proteins tropomyosin and alpha-actinin through isolation and use of poly Alpha+mRNA35 in a cell-free translational system containing S-methionine, immunoprecipitation of the product and analysis of radioactivity in the product. (3) Quantity of 3'5' cyclic adenosine monophosphate (cAMP) and 3 5 cyclic guanosine monophosphate (cGMP) by radioimmunoassay (4) Activity of the enzyme cytidine triphosphate: Phospho- choline cytidyl transferase using the enzyme catalyzed reaction (5) Apical ectodermal ridge (AER) through ablation of AER and by determining the localization of T3 in hindlimb tissue through application of scanning electron microscopy to X-ray analysis of tissue sections.